24 BRITISH FOSSIL TRIGONE. 



rarely, and does not usually much exceed the size of our figured specimen ; it is found in 

 the Calcareous Grit at Osmington Hill ; the same formation at Filey Point, Yorkshire, has 

 also produced it rarely, and not well preserved. In Normandy it is very abundant in 

 the Coral Rag of Glos and of Hennequeville. 



Trigonia ingens, Lycett, sp. nov. Plate VIII, figs. 1, 2, 3. 



Shell sub-ovate or ovately oblong, convex anteally ; umbones obtuse, moderately 

 produced ; anterior border short, curved arcuately with the lower border ; posterior 

 border nearly straight, sloping obliquely, and terminating in a rounded, wide, posteal 

 extremity ; escutcheon large, lengthened, concave, its superior border raised ; area 

 moderately large, slightly convex, with a mesial divisional furrow, bordering a small 

 medium tuberculated carina ; the area is also bounded by two small minutely tuberculated 

 carinas ; its surface has small transverse striations, which over its posteal half become 

 irregular rugose plications, the carinas at the same position also disappear. The other 

 portion of the shell has about fourteen rows of large, oblique, tuberculated costae ; the 

 tubercles, six or seven to each row, are large and rounded, but sometimes compressed at 

 their upper sides; they are nearly equal in size, but become suddenly small at the anteal 

 curvature of the valve, where the costae become cord-like and bend upwards. The last 

 formed two or three costae are smaller, more depressed, and cord-like, or without distinct 

 tubercles, and in this degenerated condition they proceed anteally in the direction of the 

 lines of growth, or nearly parallel to the lower border. Specimens of adult growth have 

 the lengthened anteal slope occupied by a series of short, narrow, ridge-like, sub-tubercu- 

 lated costae, which pass upwards almost perpendicularly to the extremities of the larger 

 costae ; there are about twelve of these supplementary costse, they gradually disappear at 

 the curvature which unites the anterior and lower borders. 



The lines of growth are strongly defined over the whole of the valves. 



This is the only British species of the Clavellata known in the Cretaceous Rocks. 

 Compared with the numerous Jurassic clavellated species, it does not appear to possess 

 any sectional distinctive features ; its nearest ally is T. Vollzii, which it closely resembles 

 in the characters of the tuberculated costse, excepting that the rows are somewhat more 

 elevated or ridge-like, and that the largest tubercles are those nearest to the marginal 

 carina ; the general figure also is less lengthened ; the umbones are more obtuse, or less 

 produced, less attenuated, and have not the curvature of the Kimmeridge Clay species ; 

 the posterior side is of greater breadth, and is without attenuation or flattening ; the 

 short anteal supplementary series of costae is also another distinctive feature. 



The internal mould is inflated anteally, compressed posteally ; the apices are widely 



